Commonly available electronic devices exist in the form of a thin sheet of semiconductor material, or die, with electronic circuitry disposed thereon by way of various photolithographic processes. To protect the circuitry from damage, the die is often enclosed in a package designed to facilitate the attachment of the electronic device to a printed circuit board.
Improvements in electrical signal characteristics, as well as increased flexibility in power consumption and die size, have been realized through the use of packages such as the “flip-chip” or “controlled collapsed chip connection” (C4) package which typically integrates a lid made of aluminum or other thermally conductive material that serves as a heatsink (also known as a “heat spreader”), and a substrate made of an organic compound that incorporates electrically conductive materials used to provide electrical connections between the die and a circuitboard to which the package is attached.
FIG. 1 is a cross-sectional of a prior art package of the ball grid array type. The exterior of package 100 is comprised of lid 110 (also known as an integrated heat spreader, or IHS), substrate 112 and sealant 114 disposed between lid 110 and substrate 112. Thermal attach 116 provides a thermally conductive connection between die 130 and lid 110, which serves to aid in cooling die 130 during normal operation of the circuitry disposed on die 130 by emitting heat conducted from die 130 through thermal attach 116 to the ambient air surrounding the exterior of lid 110. Underfill 120 and solder balls 122 (also known as C4 bumps) attach die 130 to substrate 112, with solder balls 122 providing electrical connections between die 130 and substrate 112. Substrate 112 is a printed circuit board with conductors forming electrical connections between solder balls 122 and solder balls 124. Solder balls 124 form electrical connections between substrate 112 and circuitboard 140 when package 100 is attached to circuitboard 140, and thereby, form electrical connections between die 130 and circuitboard 140.
FIG. 2 is a top side view of the same prior art package depicted in FIG. 1, with the items of package 200 of FIG. 2 being labeled with 2xx numbers that correspond to the 1xx numbers of the labeled items of package 100 of FIG. 1. What would have been a lid and thermal attach corresponding to lid 110 and thermal attach 116, respectively, of FIG. 1 have been removed from the package depicted in FIG. 2 to allow the relative position of other items under the lid to be seen. As shown in FIG. 2, the exterior of package 200 is comprised of substrate 212 and sealant 214. Die 230 is substantially centered relative to substrate 212, and is attached to substrate 212 via underfill 220 shown protruding from underneath and just beyond the edges of die 230. Sealant 214 is disposed to correspond to where the lid (not shown) would meet with substrate 212, so as to bond the lid to substrate 212. As shown, sealant 214 is disposed in a manner forming an unbroken line that surrounds die 230.
A drawback to disposing sealant 214 in an unbroken line is that a complete seal is formed between the lid and substrate 212 that can prevent differences in pressure between the interior of package 200 and the ambient air surrounding the exterior of package 200 from being equalized. This becomes especially significant where substrate 212 is made of organic or other material that is susceptible to absorbing moisture from the ambient air surrounding package 200, and then releasing moisture into the interior of package 200. During thermal testing or normal use, when the temperature of die 230 increases, moisture present within the interior of package 200 become steam, causing an increase in pressure within package 200, and tending to push the lid and substrate 212 apart. As can best be appreciated by reviewing FIG. 1, the pushing apart of lid 110 and substrate 112 tends to separate lid 110 from die 130, thereby reducing the pressure normally exerted by lid 110 to squeeze thermal attach 116 against die 130, and thereby reducing the effectiveness of thermal attach 116 in conducting heat away from die 130. This increase of pressure can also cause substrate 112 to bow outward, giving substrate 112 a curved shape that exerts stress on the corners of die 130 which may cause thin film cracking and/or thin film delamination leading to electrical failure and/or decreased lifespan of the circuitry disposed on the surface of the die.
Still another drawback arising from the presence of moisture being converted to steam under pressure, is that steam could be forced between either thermal attach 116 and die 130, or between thermal attach 116 and lid 110. This reduces the contact between thermal attach 116 and either lid 110 or die 130, and thereby also reduces the effectiveness of thermal attach 116 in conducting heat away from die 130 and towards lid 110. Furthermore, depending on the composition of thermal attach 116, steam may also penetrate the thermal attach material, itself, thereby forming gaps within thermal attach 116 which would block the transmission of heat from die 130 through thermal attach 116.
In addition to undesirable effects resulting from a buildup of pressure and the presence of moisture, there are drawbacks arising from the use of an unbroken line of sealant as a result of differences in the rates of expansion and contraction between the lid, the substrate and the die. Referring once again to FIG. 1, as temperature increases, both lid 110 and substrate 112 tend to expand more rapidly than die 130, and at different rates relative to each other, depending on the particular materials used in making lid 110 and substrate 112. An unbroken line of sealant restricts relative movement of lid 110 and substrate 112 more than sealant with a pattern of breaks would, and as a result, either lid 110 or substrate 112 can be caused to bow outwardly. As earlier described, this bowing out effect can either reduce the effectiveness of thermal attach 116 in conducting heat away from die 130, or can cause cracking and/or delamination in die 130.